I. Field of the Invention
This invention relates to wick-based controlled release devices. In particular, the invention relates to methods for reducing seepage from a wick-based controlled release device for transporting liquids, such as fragrances or insect repellants, from a reservoir to a surface exposed to the ambient air. The invention also relates to a wick-based controlled release device having reduced seepage.
II. Description of the Related Art and Problem to Be Solved
Devices that release vapors into the ambient air are well-known in the art. Generally, the purpose of these devices is to deodorize, disinfect, or impart a desired fragrance to the ambient air, or to distribute toxins into the air to kill or to repel unwanted pests, such as insects.
Several methods have been employed to dispense vapors into the air. For example, aerosol containers have been used to eject vapors into the air upon the activation of a trigger by a user. Other methods, however, utilize the evaporative properties of liquids, or of other vaporizable materials, to cause vapors with desired properties to be distributed into the ambient air. One such evaporative method utilizes a wick to deliver a vaporizable liquid from a reservoir to a surface exposed to the ambient air. From the exposed surface, the liquid is vaporized and dispersed into the ambient air. The exposed surface may be either the surface of the wick or the surface of another body in fluid communication with the wick.
Because wick-based controlled release devices have a surface exposed to the ambient air, as well as a path for vaporizable liquid to reach the exposed surface, unwanted seepage (leakage) of the liquid can be a problem. For instance, seepage of the liquid can occur when the wick-based devices are accidentally overturned, such as during packaging, shipping, or use by the consumer. The seepage can occur through the wick itself, as well as through any opening in the wick-based device, such as a vent hole provided to prevent vacuum formation in the liquid reservoir. Therefore, a need exists, in the design of wick-based controlled release devices, to reduce the amount of seepage of liquid from such devices.
It has previously been suggested that pore size and/or porosity of a wick may be selected to reduce seepage. For example, U.S. Pat. No. 4,968,487 discloses that, in order to prevent leakage from a wick in a wick-based device and to ensure that the liquid in the device transpires stably and effectively, the porosity of the wick should be restricted to a range of 25 to 40 percent. U.S. Patent Publication No. 2002/0136886 A1 discloses that a pore size of less than about 250 microns and a porosity of less than about 60 percent achieves effective control of liquid delivery with an additional benefit of reducing or preventing leakage.
It is also known in the art that the properties and evaporation rate of liquids in wick-based devices may be advantageously selected or controlled.
For example, U.S. Pat. No. 4,663,315 teaches a vaporizable composition including an organic solvent, an active ingredient, and at least one compound selected from a group of compounds listed in the patent, such as 3,5-di-t-butyl-4-hydroxytoluene, 3-t-butyl-4-hydroxyanisole, and mercaptobenzimidazole. According to the '315 patent, the composition provides efficient evaporation of the active ingredient and is capable of being drawn up a reservoir by an absorbing body, such as a wick, without clogging the absorbing body.
Further, U.S. Pat. No. 5,081,104 discloses that the solubility and evaporation rates of volatile fragrances such as perfumes are improved by the addition of 3-methyl-3-methoxy butanol or an ester thereof. Evaporation-rate-modifying solvents may be added to the composition taught by the patent, but addition of a surfactant is said to be undesirable because the surfactant would tend to clog a wick and block surface evaporation. The composition disclosed in the '104 patent may be in the form of a paste or a gel, obtained by the addition of thickeners, such as carboxymethyl cellulose.
We believe, however, that there is still room for improvement in controlling the seepage of liquid in wick-based controlled release devices.